Field of the Invention
The invention relates to bulk acoustic wave filters.
Electrical filters which are formed from bulk acoustic wave resonators or stacked crystal filters are normally referred to as bulk acoustic wave filters.
Bulk acoustic wave resonators typically include two electrodes and a piezo-electric layer, which is arranged between the two electrodes. A stack such as this, which is formed from an electrode/piezo-electric layer/electrode, is arranged on a mount that reflects the acoustic wave (M. Kenneth, G. R. Kline, K. T. McCarron, High-Q Microwave Acoustic Resonators and Filters, IEEE Transactions on Microwave Theory and Techniques, Vol. 41, No. 12, 1993).
FIG. 1 shows a cross section through a bulk acoustic wave resonator. In principle, it would be desirable to use a configuration including only an electrode 1/piezo-electric layer 3/electrode 2. However, an arrangement such as this is not sufficiently robust. The arrangement is therefore applied to a substrate 4, although this is associated with the disadvantage that the sound waves penetrate into the substrate 4, in consequence resulting in interference. The substrate 4 should at the same time provide as good acoustic isolation as possible in addition to providing a mechanical supporting function. FIG. 1 shows an acoustic mirror, which includes a substrate 4 and a sequence of two low-Z 5 layers and two high-Z 6 layers.
Stacked crystal filters in general include two piezo-electric layers and three electrodes. This total of five elements forms a sandwich structure, with one piezo-electric layer in each case being arranged between two electrodes. The central one of the three electrodes is in this case generally used as a grounding electrode.
FIG. 2 shows a cross section through a stacked crystal filter. The stacked crystal filter includes a substrate 7, a membrane 8, a first, lower electrode 9, a first, lower piezo-electric layer 10, a second, upper piezo-electric layer 11, a second, central electrode 12 and a third, upper electrode 13. The central electrode 12 is arranged above a part of the lower piezo-electric layer 10 and of the membrane 8, the upper piezo-electric layer 11 is arranged above parts of the central electrode 12 and of the lower piezo-electric layer 10, and the third, upper electrode 13 is arranged above the upper piezo-electric layer 11. The second electrode 12 is used as a grounding electrode. The substrate 7 has a cavity 14 which is used to reflect the acoustic oscillations of the piezo-electric layers.
The reflection of the acoustic oscillations is thus achieved either using an acoustic mirror or using a cavity. An acoustic mirror has been described above in conjunction with a bulk acoustic wave resonator, while the reflection of the acoustic oscillations has been shown using a cavity for a stacked crystal filter. However, the opposite combination is, of course, also possible, that is to say a bulk acoustic wave resonator in combination with a cavity in the substrate, and a stacked crystal filter in combination with an acoustic mirror.
The piezo-electric layers are generally formed from aluminum nitride. Aluminum, alloys containing aluminum, tungsten, molybdenum and platinum are frequently used as the material for the electrodes. Silicon, gallium arsenide, glass or a sheet, for example, can be used as the substrate material.
As has already been explained above, each bulk acoustic wave resonator or stacked crystal filter has at least two electrodes. FIG. 3 shows a view of two electrodes mounted one on top of the other, namely a lower electrode 15 and an upper electrode 16. The two electrodes may have any desired geometric shape. For the purposes of the present invention, the “effective resonator surface” is regarded as the surface of the electrodes which results from the overlapping area of the electrodes when the two electrodes are projected in a plane. The effective resonator surface of the electrodes 15 and 16 is illustrated in shaded form in FIG. 3. Since the electrodes 15 and 16 may in principle have any desired shape, this means that the effective resonator surface may be a planar surface of any desired shape.
Every bulk acoustic wave resonator therefore has a specific effective resonator surface, which is characterized by its geometric shape and by its surface content. Two bulk acoustic wave resonators with different effective resonator surfaces may thus differ in principle in the surface shape of the effective resonator surface and/or in the surface content of the effective resonator surface.
A bulk acoustic wave filter is composed of two or more bulk acoustic wave resonators or stacked crystal filters connected in parallel or in series. The expression “bulk acoustic wave resonator” is used synonymously for both devices in the following text, and FIGS. 1 and 2 show apparatuses used for them, namely bulk acoustic wave resonators and stacked crystal filters.
Bulk acoustic wave filters are generally designed such that the series-connected resonators have a series resonance whose frequency corresponds as precisely as possible to the desired frequency of the filter while, in a corresponding way, the parallel-connected resonators have a parallel resonance, whose frequency likewise corresponds as precisely as possible to the desired frequency of the filter.
One particular problem with the use of bulk acoustic wave filters is the spurious modes of the bulk acoustic wave resonators from which the filters are formed. These spurious modes lead to interference spikes in the electrical impedance curve of the bulk acoustic wave resonators, which then also have a disadvantageous effect on the pass band of the filters. In particular, the standing wave ratio is made worse and/or the phase curve of the filters is distorted, which, for example, infringes the condition of a constant group delay time within a transmission channel in receiver front ends.
Various approaches to the suppression of the spurious modes are known from the prior art. U.S. Pat. No. 5,903,087 discloses bulk acoustic wave resonators whose electrodes are not smoothed at the edges, but in fact, have roughened edges in the form of a random pattern, with the roughness being approximately of the same magnitude as the wavelengths of the spurious modes. The spurious modes are thus suppressed, and are less visible in the impedance curve. However, this method results in a major energy losses, which affect the Q-factor of the main resonances.